Mass spectrometry (MS) is used widely for identifying and quantifying molecular species in a sample. During analysis, molecules from the sample are introduced into the mass analyzer and are ionized to acquire a charge, thus forming ions. The analyzer responds to each of the molecule's ions by the ratio of their mass to the charge that they have acquired; hence mass-to-charge ratio m/z. A detector produces a signal relating to the intensity of the ions at their particular m/z.
A chromatographic separation technique may be performed prior to injecting the sample into a mass spectrometer. Chromatography is a technique for separating compounds, such as those held in solution, where the compounds will exhibit different affinity for a separation medium in contact with the solution. As the solution flows through such an immobile medium, the compounds separate from one another. Common chromatographic separation instruments include gas chromatographs (GC) and liquid chromatographs (LC). When coupled to a mass spectrometer, the resulting systems are referred to as GC/MS or LC/MS systems. GC/MS or LC/MS systems are typically on-line systems in which the output of the GC or LC is coupled directly to the MS.
In an LC/MS system, a sample is injected into the liquid chromatography system at a particular time, which triggers the system to acquire data. The liquid chromatography causes the sample to elute over time resulting in a separated analyte that exits the column. The time at which a particular analyte exits the column is called its retention time. The eluent exiting the liquid chromatograph is continuously introduced into the ionization source of the mass spectrometer. As the separation progresses, the composition of the mass spectrum generated by the MS evolves and reflects the changing composition of the eluent.
Typically, at regularly spaced time intervals, a computer-based system samples and records the spectrum. The intensity response of an ion is the height or area of the peak as seen in its spectrum. The spectra generated by conventional LC/MS systems may be further analyzed. Mass or mass-to-charge ratio measurement of an ion are derived through examination of a spectrum peak (intensity vs. m/z) that contains the ion. Retention time measurement of an ion are derived by examination of a chromatogram peak (intensity vs. time) that contains the ion.
Two stages of mass analysis (MS/MS also referred to as tandem mass spectrometry) may also be performed. One particular mode of MS/MS is known as product ion scanning where parent precursor ions of a particular m/z value are selected in the first stage of mass analysis by a first mass filter/analyzer. The selected precursor ions are then passed to a collision cell where they are fragmented to produce product fragment ions. The product fragment ions are then analyzed by a second mass filter/analyzer.